It is known to provide manhole cover assemblies which comprise fixed rings which can be set in a roadway surface to surround a manhole opening and plug-type covers which are adapted to rest upon inwardly extending flanges of this ring and be connected thereto by a bayonet-type connection whereby projections from the cover engage beneath projections on the ring upon rotation of the cover in one sense. When the cover is rotated in the opposite sense, the radial projections of the cover clear the generally tangential projections of the ring and enable lifting of the cover.
Problems have been encountered with such assemblies because of the inadvertent release of the cover by the ring and as a result of passage of traffic over the assembly. In addition, there has been a tendency for the radial projections of the cover to be broken off upon repeated removal and replacement of the cover and/or as a result of the passage of heavy vehicles thereover. Since the covers are particularly heavy, because they are generally composed of malleable cast iron or spheroidal graphite cast iron, this tendency is pronounced and replacement of the projections is difficult if not impossible.
Another disadvantage of conventional bayonet-locking arrangements is that the personnel responsible for replacing the cover frequency forget to lock the same in place. As a result, vibration imparted to the cover by the passing traffic tends to induce pivoting of the cover and the release thereof with the ensuing danger to the vehicles and passers-by.
It is known to provide supplementary locking arrangements which are intended to avoid inadvertent opening of the bayonet connection, e.g. by requiring the operation of a key which is rotatable about a vertical axis and which carries a plate closing the chamber through which the key passes and which can be formed as an eye on the ring.
This key can be mislaid in handling of the cover so that, when the cover is replaced, it cannot be locked.
Other problems are also encountered with such existing structures, namely, that of lifting the cover conveniently, preventing loss of the key and ensuring that locking will be effected after replacement of the cover in the ring following its removal.